MRI - Fast Spin Echo Spin Echo • Difference between multiecho SE and FSE ... • Directly...

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Bioe594-1/24/2006 Fast Spin Echo BioE 594 – Advanced Topics in MRI Nick Gruszauskas

Transcript of MRI - Fast Spin Echo Spin Echo • Difference between multiecho SE and FSE ... • Directly...

Page 1: MRI - Fast Spin Echo Spin Echo • Difference between multiecho SE and FSE ... • Directly determines the amount that scan time is reduced when compared to conventional SE.

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Fast Spin Echo

BioE 594 – Advanced Topics in MRINick Gruszauskas

Page 2: MRI - Fast Spin Echo Spin Echo • Difference between multiecho SE and FSE ... • Directly determines the amount that scan time is reduced when compared to conventional SE.

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Review

• Spin Echo Pulse Sequence– 90° RF excitation 180° RF refocus echo– Main parameters: TR and TE– Can generate T1, T2, and proton-density

images

( )2112

0 21 TTE

TTRTTETR eeeMS

−−−−⎟⎠⎞

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Page 3: MRI - Fast Spin Echo Spin Echo • Difference between multiecho SE and FSE ... • Directly determines the amount that scan time is reduced when compared to conventional SE.

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Review

• Spin Echo Pulse Sequence

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Review

• Advantages of SE– Minimal image artifacts– Relatively high SNR– 180° refocusing pulse provides:

• Elimination of signal loss from field inhomogeneities, susceptibility effects, etc.

• Signal decay that is only caused by T2, not T2*

Page 5: MRI - Fast Spin Echo Spin Echo • Difference between multiecho SE and FSE ... • Directly determines the amount that scan time is reduced when compared to conventional SE.

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Review

• Disadvantages of SE– Moderate to high SAR– Relatively long TRs and acquisition times

Page 6: MRI - Fast Spin Echo Spin Echo • Difference between multiecho SE and FSE ... • Directly determines the amount that scan time is reduced when compared to conventional SE.

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Moving Beyond Spin Echo• In conventional spin echo, data is acquired for

only a small fraction of the lifetime of the transverse magnetization created by the refocusing pulse– Ex: in a system with a readout matrix size of 256 and

a receiver bandwidth of ±16 kHz, the acquisition time is 256×(1/(2×16)) = 8 ms per TR. The length of the actual spin echo is governed by T2 relaxation time, which is roughly 100 ms for many tissues.

• A considerable amount of signal that could be sampled is left to decay after acquisition – can this signal be utilized to our advantage?

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Moving Beyond Spin Echo

• Multiecho Spin Echo– Obtains multiple echoes simultaneously

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Moving Beyond Spin Echo

• Multiecho Spin Echo– Multiple echoes collected can be used to

calculate T2

– No increase or decrease in acquisition time from single-echo SE

• Acquisition time: can it be improved without loss in image quality?

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Fast Spin Echo• Rapid Acquisition with Relaxation Enhancement

(RARE), also known as Fast Spin Echo (FSE)

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Fast Spin Echo

• A train of refocusing pulses follows the initial excitation pulse

• Multiple spin echoes are produced by the multiple refocusing pulses

• Each echo is distinctively encoded with a phase-encoding gradient

• Multiple k-space lines can be sampled following each excitation pulse

Page 11: MRI - Fast Spin Echo Spin Echo • Difference between multiecho SE and FSE ... • Directly determines the amount that scan time is reduced when compared to conventional SE.

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Fast Spin Echo• Difference between SE and FSE:

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Fast Spin Echo• Difference between multiecho SE and FSE

– Multiple phase encodes per TR in FSE– Only one phase encode per TR in multiecho SE

Multiecho SE FSE

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Fast Spin Echo• Signal intensity at the nth echo is determined by the

conventional Bloch description as well as some additional parameters:

– where M0 is the equilibrium magnetization, T1 is the longitudinal relaxation time, TEn = 2nτ is the nth echo time (and τ is the time interval between the excitation pulse and the first refocusing pulse), and T2 is the transverse relaxation time

– TD is the time interval between the center of the last echo in thetrain and the end of the TR and is given by TD = TR - 2Nτ, where TR is the repetition time and Nτ is the total number of refocusing pulses

2110T

TETT

n

nD

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Fast Spin Echo• In considers the intensity produced at acquisition, which

is derived from transverse magnetization but is also dependent on the longitudinal magnetization that has recovered at the end of the previous TR period

• For an FSE sequence, the longitudinal magnetization Mzat the end of a TR period immediately prior to sequence repetition becomes:

– where the F term is a geometric series that describes the intensity of multiple echoes separated by 2τ (in a first-order approximation where τ << T1, F≈1).

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FeMM

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Fast Spin Echo• Reduction in image acquisition time

– In this example, 4 echoes are used to fill the entire k-space during a single TR. This takes approximately 1/4th as much time as 4 single conventional spin echoes would.

k-spacediagram

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FSE – Filling k-space• k-space is essentially divided into “sections”• Each echo contributes to one particular section

of k-space because it has been distinctively spatially encoded

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FSE – Basic Parameters• Echo Train Length (ETL)

– The number of echoes that follows an excitation pulse (“echo train”).

• Effective TE (TEef)– The TE of the echo acquired with a phase encoding of zero.

Represents the center line of k-space.• Number-of-Averages (NEX or NSA)

– Number of signal averages used.• Echo Spacing (ESP)

– The interval between the peaks of two consecutive spin echoes.• Number-of-Shots (Nshot)

– Number of non-redundant RF excitations. In single-shot imaging, the entire k-space for an image is acquired using the signal acquired from an echo train that was produced using a single RF excitation pulse.

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Echo Train Length• Directly determines the amount that scan time is

reduced when compared to conventional SE.• If all echoes are distinctively phase-encoded (as

with FSE) then ETL is equal to the number of k-space lines acquired during a single TR.

• The larger the ETL, to greater the scan time reduction: more lines of k-space are acquired per TR.

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Echo Train Length• The ETL has practical limitations (like T2 relaxation

times).• A larger ETL does not always translate into a large

reduction in acquisition time:

In this example, the TEef is defined by the last echo when ETL is 4.

When the ETL is 8 the TEefremains the same but only half as many images can be obtained during a single TR.

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Effective TE

• Each echo has its own TE value• The TE of an FSE sequence is defined as

the TE when the central k-space line is acquired and is known as the TEef(effective TE).– Minimal TEef value: TEef = ESP– Maximal TEef value: TEef = ETL × ESP

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Echo Spacing• If the ESP is

minimized the ETL can be long for a given TR.

• This decreases the number of excitations that must be repeated.

Page 22: MRI - Fast Spin Echo Spin Echo • Difference between multiecho SE and FSE ... • Directly determines the amount that scan time is reduced when compared to conventional SE.

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Determining Scan Time• The total scan time (where Nacq is the number of

acquisitions) of an FSE sequence can be calculated by:

• The length of a single FSE sequence can be approximated by:

acqshotscan NNEXNTRT ⋅⋅⋅=

ESPETLTseq ⋅≈

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Sequence Efficiency

• Multiple echoes are acquired during a given TE

• Efficiency increases by a factor equal to the number of echoes obtained by the time TEef is reached– Ex: if 6 echoes can be obtained in 90 ms, an

FSE technique with an echo train of 6 and a TEef of 90 ms is six times as efficient as a single-echo technique with a TE of 90 ms.

Page 24: MRI - Fast Spin Echo Spin Echo • Difference between multiecho SE and FSE ... • Directly determines the amount that scan time is reduced when compared to conventional SE.

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Contrast and SNR• Assuming 180° refocusing pulses, the peak amplitude of

the echoes decays according to:

– where n is the echo index, ESP is the echo spacing, and S0 is the peak signal value

• To produce T1-weighted or PD-weighted images, early echoes in the train are sampled for the central k-space data

• To produce T2-weighted images, later echoes are sampled for the central k-space data

( ) 20

TESPn

eSnS⋅−

=

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Contrast and SNR

• FSE demonstrates higher SNR and image quality when compared to conventional SE imaging– SNR becomes a function of TEef, ETL, and

ESP– Images are less sensitive to off-resonance

effects

Page 26: MRI - Fast Spin Echo Spin Echo • Difference between multiecho SE and FSE ... • Directly determines the amount that scan time is reduced when compared to conventional SE.

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Homework Problem #1• Consider a spin system with an average T2

relaxation time of 100 ms. If spin echoes are acquired prior to the time when the transverse magnetization decays to 20% of its peak value, what is the maximal ETL that can be acquired when ESP = 8 ms? If ESP is shortened to 4 ms by reducing the refocusing pulse width, what is the new maximal ETL?– Hint: recall that the transverse magnetization decays

according to and that t = ETL × ESP.20

TteSS −=

Page 27: MRI - Fast Spin Echo Spin Echo • Difference between multiecho SE and FSE ... • Directly determines the amount that scan time is reduced when compared to conventional SE.

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Clinical Use• Often used to produce T2-weighted images• Contrast is very similar to conventional SE• Scan parameters usually include TR, TE, ETL

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Single-shot vs Multi-shot

• In single-shot FSE, a single echo train is used to acquire all the k-space data needed for the reconstruction of an image.

• Commonly used to compensate for respiratory motion or improve temporal resolution.

• ETL must be maximized to achieve acceptable spatial resolution.

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Single-shot vs Multi-shot• In multi-shot FSE, a

fraction of the k-space data is acquired with each shot.

• Maximum ETL does not need to be used, allowing for higher spatial resolution.

• Interleaving k-space filling is usually used to avoid signal decay issues and phase errors.

Page 30: MRI - Fast Spin Echo Spin Echo • Difference between multiecho SE and FSE ... • Directly determines the amount that scan time is reduced when compared to conventional SE.

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CPMG Conditions

• Refocusing pulses with flip angles of less than 180° are often used

• As a result, some transverse and longitudinal magnetization remains after refocusing (imperfect refocusing)

• Subsequent non-180° pulses further differentiate these imperfect magnetization components

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CPMG Conditions• The “interference” of these incomplete

magnetizations causes the creation of primary, secondary, and stimulated spin echoes.

• Secondary and stimulated echoes are the result of excitation by imperfect refocusing pulses

• The pulses create echoes from “lingering”magnetization, which artificially increases the total number of echoes.

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CPMG ConditionsExcitation RF pulse

Refocusing RF pulse Spin echo

FIDPrimary echoStimulated echoSecondary echo

Legend

RF PulseTrain

Signal Phase

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CPMG Conditions• In order to ensure that

– Echoes only occur at the desired points in the pulse sequence

– The signals at each temporal position have the same phase

• The Carr-Purcell-Meiboom-Gill (CPMG) conditions must be met for an FSE sequence

• When these conditions are met, primary and stimulated echoes occur only at the mid-point between two consecutive refocusing pulses and carry the same phase

Page 34: MRI - Fast Spin Echo Spin Echo • Difference between multiecho SE and FSE ... • Directly determines the amount that scan time is reduced when compared to conventional SE.

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CPMG Conditions

• Condition 1– Refocusing pulses must be 90° out-of-phase

with excitation pulses– Refocusing pulses must be evenly positioned

with equal spacing between any two consecutive pulses.

– The spacing of the refocusing pulses must be twice the time interval τ between the excitation pulse and the first refocusing pulse

Page 35: MRI - Fast Spin Echo Spin Echo • Difference between multiecho SE and FSE ... • Directly determines the amount that scan time is reduced when compared to conventional SE.

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CPMG Conditions

• Condition 2– Phase accumulated by a spin isochromat

between any two consecutive refocusing pulses must be equal

( ) ( ) ( )dttBdttBdttB N

N

t

t

t

t

t

t ∫∫∫−

==1

4

3

2

1

γγγ L

Page 36: MRI - Fast Spin Echo Spin Echo • Difference between multiecho SE and FSE ... • Directly determines the amount that scan time is reduced when compared to conventional SE.

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CPMG Conditions• Implications of CPMG conditions in FSE

– All crusher gradients surrounding the refocusing pulses must have the same area (with some exceptions)

– Each phase-encoding gradient must be accompanied by a phase-rewinding gradient after the readout window but prior to the next refocusing pulse

– Only primary and stimulated echoes will be used; secondary echoes and the FIDs following each refocusing pulse are eliminated by crusher gradients

– If a non-uniform space exists in the magnetization along the slice-selection direction, the CPMG conditions cannot be satisfied across the entire slice profile

– Moving spins can violate the CPMG conditions (accumulated phase would no longer be equal)

Page 37: MRI - Fast Spin Echo Spin Echo • Difference between multiecho SE and FSE ... • Directly determines the amount that scan time is reduced when compared to conventional SE.

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Advantages of FSE• Short acquisition time• Image contrast similar to SE

– Refocusing pulses eliminate signal loss from field inhomogeneities and susceptibility effects when compared to gradient echo-based techniques

• High spatial resolution and SNR– Can be adjusted at the expense of scan time

• Less signal loss in adipose tissue due to “J coupling” when compared to SE– J coupling is loss due to interaction between different

lipid resonances during gaps between refocusing pulses

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Disadvantages of FSE• Larger power deposition/SAR• Larger magnetization transfer (MT) effects

– MT involves saturation of the longitudinal magnetization of macromolecular protons, which can be transferred to closely-associated water protons

• J coupling can still effect scans with long TR and TE– Lipids may appear brighter and obscure

potential lesions

Page 39: MRI - Fast Spin Echo Spin Echo • Difference between multiecho SE and FSE ... • Directly determines the amount that scan time is reduced when compared to conventional SE.

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Disadvantages of FSE• Blurring and ghosting

– Produced by motion, T2effects, and phase-induced artifacts

• Edge enhancement– Some tissues with

blurred edges are mapped to adjacent pixels, increasing apparent signal intensity (usually at fluid-tissue interfaces)

Page 40: MRI - Fast Spin Echo Spin Echo • Difference between multiecho SE and FSE ... • Directly determines the amount that scan time is reduced when compared to conventional SE.

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Disadvantages of FSE• Many of these disadvantages can be corrected

or compensated:– Refocusing pulses of less than 180° for lower SAR

(also reduces MT effects)– HASTE and SSFSE techniques to reduce blurring– T2 mapping, averaging, or weighting can reduce

blurring and edge enhancement– Phase compensation to reduce ghosting– Careful selection of TR, TE, ETL, and using different

prepatory pulses like fat saturation for improved image quality

Page 41: MRI - Fast Spin Echo Spin Echo • Difference between multiecho SE and FSE ... • Directly determines the amount that scan time is reduced when compared to conventional SE.

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Homework Problem #2

• Consider an FSE sequence where Nacq = 2, ETL = 64, and the number of phases is 64. If the ETL is reduced to 32 what must be done to the sequence in order to acquire the same number of phase-encoded k-space lines? What happens to the sequence in general?– Hint: How does Nshot change and what

happens to Tseq?

Page 42: MRI - Fast Spin Echo Spin Echo • Difference between multiecho SE and FSE ... • Directly determines the amount that scan time is reduced when compared to conventional SE.

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References and Figures from:1. Bernstein, M.A.; King, K.F.; Zhou, X.J., Handbook of MRI Pulse Sequences.

Elsiver/Academic Press: Burlington, MA, 2004.2. Mitchell, G.M.; Cohen, M.S., MRI Principles. Elsiver/Saunders: Philadelphia,

PA, 2004.3. Hornak, J.P., The Basics of MRI. Online, 2005.

<http://www.cis.rit.edu/htbooks/mri/index.html>4. Hennig, J.; Nauerth, A.; Friedburg, H., “RARE imaging: a fast imaging method

for clinical MR.” Magn Reson Med 1986, 3, (6), 823-833.5. Constable, R.T.; Anderson, A.W.; Zhong, J.; Gore, J.C., “Factors influencing

contrast in fast spin-echo MR imaging.” Magn Reson Imaging 1992, 10, (4), 497-511.

6. Constable, R.T.; Smith, R.C.; Gore, J.C., “Signal-to-noise and contrast in fast spin echo (FSE) and inversion recovery FSE imaging.” J Comput Assist Tomagr 1992, 16, (1), 41-47.

7. Meara, S.J.; Barker, G.J., “Evolution of the longitudinal magnetization for pulse sequences using a fast spin-echo readout: application to fluid-attenuated inversion-recovery and double inversion-recovery sequences.”Magn Reson Med 2005, 54, (1), 241-245.